1984 Targa tune up problems

[Switzer Deason]

I took my 1984 Targa to a local (reputable) garage for a general check up, and they recommended a complete tuneup (spark plugs, air hose, reference sensor, oxygen sensor, distributor cap & rotor, ignition coil, coil wire wire, etc) -- for a cost of $1,800 or so -- the car was running great, but it hadn't been tune in 6 years and 10,000 highway miles. However . . . after the tuneup the engine stammers and they can't find it. I didn't have this problem before they replaced everything having to do with a tune up. In 1st and 2nd, the engine basically stammers and stutters at 4,000 rpm or so. At 2,500 rpm and the related hwy speed of 60, it stammers. At 3,000 rpm and 70 mph, it runs ok, unless i start up a small incline and the engine begins to stammer. at 4,000 rpm and 110 mph, the engine runs ok while it is accelerating, but then begins to stammer at 4,000.

Since the engine was not doing any of this before the $1,800 tune up, and the garage can't determine what the cause of the problem is (they've had it for three weeks) . . . I'd greatly appreciate any input. Thanks!